Add PHPDocs for count(), getIterator(), and jsonSerialize()

[mustanggb]

Symfony is giving deprecation warnings, should be able to resolve these by adding the missing PHPDocs.

[deprecation] User Deprecated: Method "Countable::count()" might add "int" as a native return type declaration in the future. Do the same in implementation "XeroAPI\XeroPHP\Models\Accounting\Invoices" now to avoid errors or add an explicit @return annotation to suppress this message.
[deprecation] User Deprecated: Method "IteratorAggregate::getIterator()" might add "\Traversable" as a native return type declaration in the future. Do the same in implementation "XeroAPI\XeroPHP\Models\Accounting\Invoices" now to avoid errors or add an explicit @return annotation to suppress this message.
[deprecation] User Deprecated: Method "JsonSerializable::jsonSerialize()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "XeroAPI\XeroPHP\Models\Accounting\Invoices" now to avoid errors or add an explicit @return annotation to suppress this message.